(i) Field of the Invention
The present invention relates mainly to wood-cased color pencil leads and color pencil leads for mechanical pencils. More specifically, it relates to non-baked color pencil leads which is remarkably excellent in physical properties such as bending strength and pointed end strength and which has a smooth writing performance. Furthermore, it relates to color pencil leads and a method for preparing the same by which the conventional color pencil leads which cannot be erased by an eraser or the color pencil leads which can be erased by the eraser can be selectively prepared and which permits noticeably shortening a drying time and decreasing a manufacturing cost.
(ii) Description of the Prior Art
Conventional non-baked color pencil leads can be prepared by a wet process which comprises, if necessary, dissolving and swelling an organic polymeric material as a binder in a solvent such as water and/or a plasticizer, kneading the same with a wax, a filler, a coloring agent and the like, extruding the kneaded material, and then drying the extruded articles to remove the solvent and/or the plasticizer therefrom.
In this wet process, however, it is necessary that a drying temperature is set to the melting point or less of the wax or the like so as to prevent the bleeding of the wax or the like, and therefore a tremendous period of time is taken in the drying step. Moreover, leads prepared by a dry process, in which the solvent and/or the plasticizer is not used and the drying step is omitted, are also put on the market, but the quality of these leads is inferior to that of the leads obtained by the wet process.
In connection with the strength and the writing performance of the conventional color pencil leads obtained by the wet process, the adhesion of the filler and coloring agent by the binder is partially impeded with the wax or the like, so that the pencil leads are worn at the drawing and exert lubricating properties and a staining power.
However, when the wax or the like is kneaded with a blend composition containing the filler, the coloring agent, the binder and the like, followed by extruding, the effects of the smooth writing performance and the excellent staining power are remarkable, but the deterioration of strength is also large. In consequence, any satisfactory color pencil leads balanced between the strength and the writing performance have not been obtained so far.
On the other hand, the non-baked color pencil leads which are said to be erasable with an eraser are also put on the market, and most of these leads contain the binder, which is a non-polar organic material, and the wax or the like which is a non-polar organic material as the main components in order to be easily removed from the fibers of a paper. In addition, the staining power is increased by softening the leads so as to permit drawing under weak pencil pressure, whereby the lead composition is prevented from getting into between the fibers of the paper at the drawing to obtain erasability.
However, the coloring agent tenaciously sticks between the fibers of the paper at the drawing by the wax or the like, and the lead composition which has once stuck between the fibers of the paper cannot be removed completely by the adsorbing power of an eraser. Thus, this erasability is not practically satisfactory.
The removal of the drawing lines by the adsorbing power of the eraser can be achieved by using the oil, the fat and/or the wax which is liquid at ordinary temperature for all the material of the wax or the like.
However, in the conventional wet process, the drying step is effected by heating at 50.degree.-60.degree. C., and therefore the oil, the fat and/or the wax, which is liquid at ordinary temperature, bleeds and the shape of the leads cannot be maintained any more in the drying step. In consequence, the conventional wet process cannot practically utilize such an oil, fat and/or wax, which is liquid at ordinary temperature.
Thus, it can be conceived that color pencil porous leads are first prepared, and they are then impregnated with the oil, the fat and/or the wax which is liquid at ordinary temperature.
In order to form the open pores in the pencil porous leads, some techniques have been heretofore suggested which are a technique of adding a sublimable material or a pyrolysis material, and techniques such as an acid and an alkali solution treatment. However, in every technique, the heating is carried out at more than the thermal deterioration temperature of an organic pigment, and so the color pencil porous leads having a vivid color cannot be obtained.
The baked color pencil leads are only put on the market which can be prepared by kneading a clay, a white filler, a binder and the like, extruding the kneaded material, baking the extruded articles in an oxygen atmosphere to form baked white pencil porous leads, and then impregnating them with an ink. With regard to these baked color pencil leads, their erasability with the eraser is substantially satisfactory, but the impregnation is carried out by the use of a dye ink, since it is impossible to impregnate the porous leads with a highly concentrated pigment ink. This kind of leads have the problem that they are very poor in light resistance and staining power as compared with the non-baked pencil color leads in which the pigment is used.